Anatomically, the sentinel lymph node can be defined as the first node that receives lymphatic drainage from a primary tumor. If whether tumor cells in the sentinel lymph node are benign or malignant is determined, whether other lymph nodes contain metastatic tumor can be determined. Thus, if whether the sentinel lymph node contains metastatic tumor cells is determined, the status of other lymph nodes can also be predicted.
In other words, if the sentinel lymph node contains no tumor cells, it can appear that other lymph nodes also contain no tumor cells. Methods for detecting the sentinel lymph node include a method that uses isosulfan blue dye (isosulfan blue dye, Lymphazurin 1%, Ben Venue Lab., Inc., USA) as a visual aid during a surgical operation, and a method utilizing a hand-held gamma probe, which comprises injecting technetium-99m colloid albumin into a patient before a surgical operation, and then identifying radioactive lymph nodes with a Neoprobe 1500 (Neoprobe Corp, USA). When the two methods are used in combination, the sentinel lymph node can be almost completely identified. Thus, a surgical technique of identifying the sentinel lymph node and predicting the metastasis of tumor cells to other lymph nodes based on the identified sentinel lymph node has been increasingly used.
In the case of superficial cancers, like melanoma or breast cancer, when the sentinel lymph node is identified and applied surgically, unnecessary lymph node excision can be avoided. Wide local excision or radical vulvectomy, including unilateral or bilateral lymphadenectomy, is a palliative surgical method that is performed in vulvar cancer patients, and is often accompanied by sequelae such as wound breakdown, lymphoedema or lymphoma. Thus, this surgical technique is not preferable in terms of the patient's life quality, even though it is successful.
However, if the sentinel lymph node is identified and a frozen biopsy thereof is determined to be benign after whether other lymph nodes contain metastatic tumor cells is determined based on the frozen biopsy, a surgical operation can be achieved by exercising only the sentinel lymph node. This method can appear to be a highly advanced method for surgery of vulvar cancer.
However, conventional methods for detecting the sentinel lymph node using radiopharmaceuticals have many problems. Specifically, the contamination of an analyzer and an analysis room during the pathological analysis of a lymph node labeled with a radiopharmaceutical, and the disposal of a sample after pathological analysis become serious problems. Particularly, there is an urgent need for development of a method of detecting the sentinel lymph node using molecular imaging methods other than nuclear medicine methods so as to minimize the radiation exposure of patients.
In an attempt to overcome these problems, the present inventors have made efforts to develop a novel method capable of effectively detecting the sentinel lymph node using a complex of a biocompatible polymer and a biocompatible optical imaging probe.
Indocyanine Green (ICG) is a typical biocompatible optical imaging probe showing fluorescent characteristics in a near-infrared region (700-1300 nm) and is a unique near-infrared fluorescent probe approved for human use by the FDA. Near-infrared rays have advantages over visible rays in that they have high skin permeability, and thus make it possible to obtain molecular images of subcutaneous blood vessels or organs or can provide guidance to sensitive portions such as nerves or blood vessels during a surgical operation.
Indocyanine green (ICG) is used mainly in liver function tests in which ICG is injected intravenously and whether ICG remains in blood is examined at a specific time after injection. ICG binds to serum protein, is absorbed selectively by the liver, and is excreted into bile without being excreted into the kidneys. Thus, in this test method, liver functions can be examined by measuring the retention rate or removal rate of the ICG dye in blood. A retention rate of 5% or less at 15 min is a normal value, and a retention rate of 10% or more means a severe impairment in liver function caused by cirrhosis or the like.
Although ICG was used in studies on the detection of the sentinel lymph node, but there were problems in that ICG has a too small size, and thus when it is injected in vivo, it does not remain in the sentinel lymph node for a long period of time and migrates to other surrounding lymph nodes. This makes it difficult to distinguish the sentinel lymph node from lymph nodes around cancer tissue during an actual surgical operation.
Recently, a patent application was made, which is related to polymer particles containing a near-infrared fluorescent dye, which are prepared by dispersing an aqueous solution of the near-infrared fluorescent dye indocyanine green (ICG) in an organic polymer solution and dispersing the resulting dispersion in an aqueous emulsifier solution (Korean Patent Laid-Open Publication No. 2009-0026642). Such polymer particles are nano/micro-sized particles having a particle diameter of more than 100 nm can be used mainly for intravenous administration and oral administration, but are not suitable for the detection of the sentinel lymph node, which requires a particle diameter of 10-25 nm.
In addition, a patent application was made, which is related to cell-encapsulated microcapsules containing indocyanine green, which are prepared by the ionic reaction of the biocompatible anionic polymer sodium alginate with the multiple cationic polymer poly-L-lysine solution (Korean Patent Laid-Open Publication No. 2009-0008838). However, such polymer particles are not suitable for the detection of the sentinel lymph node, because they have a particle diameter of more than 10 μm for cell encapsulation.
In addition, US Patent Publication No. 20100047356 discloses microcapsules prepared by encapsulating indocyanine green by the ionic reaction of a positively charged polyelectrulyte such as poly allylamine hydrochloride and a negative charged ion such as disodium, followed by coating with silica nanoparticles. Such polymer microcapsules have a particle diameter of 0.6-2 μm and are used in photodynamic therapy based on the characteristics of indocyanine green, but are not suitable for the detection of the sentinel lymph node.
Accordingly, the present inventors have made extensive efforts to develop a molecular imaging probe which, when injected into blood, remains in the sentinel lymph node for an extended period of time and has a low tendency to migrate to other lymph nodes. As a result, the present inventors have prepared a complex of poly-gamma-glutamic acid (γ-PGA) and indocyanine green and have found that, when the complex is used as a molecular imaging probe, the sentinel lymph node can be detected in a more easy and accurate manner, thereby completing the present invention.